The present invention relates to a 4-wheel driving transfer apparatus having a wet multi-plate clutch mechanism which is hydraulically driven and, more particularly, to a 4-wheel driving transfer apparatus in which a clutch hydraulic pressure is generated by an electric oil pump.
Hitherto, as a 4-wheel driving transfer apparatus which is used in combination with an automatic transmission, for instance, there has been known an apparatus as shown in FIG. 1.
In FIG. 1, reference numerals 10, 12, and 14 denote casings. A main shaft 16 is rotatably supported to the casings 12 and 14. Reference numeral 18 denotes a planetary gear mechanism and 20 indicates a switching mechanism for switching the planetary gear mechanism 18 to two stages of a high speed and a low speed. Reference numeral 22 indicates a hydraulic clutch mechanism which can absorb a differential rotation between the front wheels and the rear wheels and is hydraulically driven. Reference numeral 24 denotes a drive sprocket which is coupled to the hydraulic clutch mechanism 22. Reference numeral 26 represents a driven sprocket which is coupled to the drive sprocket 24 through a chain 28. Reference numeral 30 denotes a front drive shaft which is formed integrally with the driven sprocket 26.
A rotational force from the automatic transmission arranged in front of the transfer apparatus is switched to a low speed (deceleration) or a high speed (direct coupling) in accordance with the switching state of the planetary gear mechanism 18 which was selected by the switching mechanism 20 and is transferred to the main shaft 16 and, further, drives the rear wheels (not shown). On the other hand, in the 4-wheel driving state in which the hydraulic clutch mechanism 22 is hydraulically driven, the rotational force of the main shaft 16 is transferred to the drive sprocket 24 through the hydraulic clutch mechanism 22 and is transferred from the drive sprocket 24 to the front drive shaft 30 through the chain 28 and driven sprocket 26. Further, a rotational force of the front drive shaft 30 is transferred to the front wheels and the 4-wheel driving running state is obtained.
However, in such a conventional transfer apparatus, the hydraulic clutch mechanism 22 for variably changing a torque distribution of the front and rear wheels is used upon switching between the 2-wheel driving mode and the 4-wheel driving mode and upon 4-wheel driving. However, since the hydraulic clutch mechanism 22 always needs the hydraulic pressure, only a combination with the automatic transmission in which a line pressure is always obtained is satisfied.
That is, in a combination with a manual transmission, there is a problem such that even if an oil pump which receives the rotation of the transmission and generates a hydraulic pressure is provided, no hydraulic pressure is obtained when the vehicle stops, and the hydraulic pressure lacks in the low-speed running mode or the reverse mode, and the switching between the 2-wheel/4-wheel driving modes by the hydraulic clutch mechanism and the control to variably change the torque in the 4-wheel driving mode cannot be properly executed.